Ferroelectric memory using reference charge circuit

ABSTRACT

A random access memory circuit is described which uses single ferroelectric memory cells to store data. The ferroelectric memory cells can be selectively read using a reference voltage which is compared to a voltage representative of data stored on the memory cell using a multiplexed sense amplifier. The reference voltage is generated using a non-remnant capacitor circuit coupled to a bit line. In using a non-remnant capacitor circuit, a single-ended reference voltage can be generated on the bit line. The capacitance of the bit line is substantially greater than the capacitance of the non-remnant capacitor, therefore, the resultant reference voltage on the bit line remains relatively constant with fluctuations in supply voltage. A ferroelectric memory cell can then be read by comparing the voltage on its corresponding bit line to the reference voltage using the sense amplifier.

This invention was made with government support under Contract No.MDA972-94-C-0006 awarded by Advanced Research Projects Agency (ARPA).The Government has certain rights in this invention.

TECHNICAL FIELD OF THE INVENTION

The present invention relates generally to ferroelectric memories and inparticular the present invention relates to a circuit which produces areference voltage used to read the memory.

BACKGROUND OF THE INVENTION

Placing ferroelectric material between the plates of a capacitor on asemiconductor substrate causes the capacitor to exhibit a memory effectin the form of charge polarization between the plates of the capacitor.In effect, when the capacitor is charged with the field lines running inone direction across the capacitor plates, a residual chargepolarization remains after the charge is removed from the capacitorplates. If an opposite charge is placed on the capacitor plates, anopposite residual polarization remains. A plot of the applied fieldvoltage (E) across the plates of the capacitor against the polarization(P) of the ferroelectric material between the plates of the capacitorexhibits a classic hysteresis curve as shown in FIG. 1. This type ofhysteresis response of ferroelectric material between the plates of thecapacitor manufactured on a semiconductor die as known in the art and isdescribed in U.S. Pat. No. 4,873,664 to Eaton Jr., which is incorporatedherein by reference.

Using ferroelectric material in the manufacture of capacitors for use inthe cells of memory arrays is also known in the art. By applying acoercive voltage across the plates of the ferroelectric capacitor toproduce one polarization or another, the residual polarization stores anonvolatile 1 or 0 in the cell. If a ferroelectric capacitor has zerovolts applied across its plates, it may be polarized as indicated byeither point A or point D in FIG. 1. Assuming that the polarization isat point A, if a positive voltage is applied across the capacitor whichis greater than the "coercive voltage" indicated by line B, then thecapacitor will conduct current and move to a new polarization at pointC. When the voltage across the capacitor returns to zero, thepolarization will remain the same and move to point D. If a positivevoltage is applied across the capacitor when it is polarized at point D,the capacitor will not conduct current, but will move to point C. It canbe seen that a negative potential can be used to change the polarizationof a capacitor from point D to point A. Therefore, points A and D canrepresent two logic states occurring when zero volts are applied to thecapacitor and which depend upon the history of voltage applied to thecapacitor.

The reading of the polarization of the ferroelectric capacitor can be adestructive read in which a pulse is applied to the ferroelectriccapacitor and the amount of resultant charge is either low if the pulsepolarity agreed with the previous memorization polarity, or theresultant charge is higher if the charge polarity placed on thecapacitor is of the opposite polarity last placed across the plates ofthe capacitor. This minute difference between an agreeable charge and anopposite charge can be measured to determine what the previouspolarization on the ferroelectric capacitor was as it was last written.If a large charge results from reading a memory cell, the memory cellpolarization will move from one state to the other state, for examplepoint A to Point D. Thus, the data read from the memory cell must berestored.

The fact that the ferroelectric capacitors require a destructive read todetermine the last polarization, and the fact that the resultant chargedifferences of the ferroelectric capacitor between an agreeable appliedpulse and an opposite applied pulse make the technique of reading andwriting ferroelectric memories a difficult task. The benefit of having anonvolatile memory in which stored data remains without any batterybackup or other external application of power is of great use in thecomputer and control industries. However, for any such nonvolatilememories to be of any use, the memories must be of a high enough densityand must have a fast enough response time to make them commercially moreattractive than battery backed up DRAM, mechanical disk storage andother types of nonvolatile storage.

One of the shortcomings of the prior art is the fact that theferroelectric capacitors age through use, producing distinctly nonlinearhysteresis curves such as that shown in FIG. 2. Thus, it becomesincreasingly difficult to determine the correct polarization of thecells as they age. For example, if a memory cell fabricated as aferroelectric capacitor is polarized at point A in FIG. 2, a positivevoltage greater than the coercive voltage B will move the polarizationof the cell to point C. When moving from point A to point C thecapacitor will conduct current. When reading a memory cell havingpolarity of point D using a positive voltage, however, a current is alsoconducted as the polarity moves to point C. The differences between theresultant currents of the two different states of the capacitor,therefore, becomes smaller as the capacitor ages. It will be appreciatedthat reading a memory cell having a hysteresis curve of FIG. 2 will bemore difficult than reading a memory cell having an ideal hysteresiscurve of FIG. 1.

Another shortcoming of the prior art is the inability to produce highdensity ferroelectric memories having high operating speeds comparableto that of DRAM storage devices. Along these lines, the aforementionedEaton Jr. patent describes the application of ferroelectric capacitorsto high density memory storage, as shown in FIG. 3. In this arrangement,each memory storage cell comprises a pair of ferroelectric capacitorsand a pair of access transistor. One plate of the pair of ferroelectriccapacitors is connected to a plate line, while the other plates of theferroelectric capacitor are connected through access transistors toseparate bit lines. In operation, a momentary voltage pulse is placed onthe ferroelectric capacitors between the bit lines and the plate line topolarize the ferroelectric material of the two ferroelectric capacitors,resulting in a polarization of one direction for one capacitor and anopposite polarization for the second ferroelectric capacitor.

Eaton Jr. takes this concept a step further by using a regular array offerroelectric capacitors, whereby each cell contains two ferroelectriccapacitors and two access transistors. The ferroelectric capacitorswithin each memory cell receives complementary input signals such thatthe ferroelectric capacitors are polarized in opposite states toindicate a 1 or a 0. When the pairs of capacitors for each cell areread, a resulting voltage on the bit lines, which result from applying apulse on a plate line, is compared using a differential sense amplifierto compare the voltages on the bit lines and thus determine the polarityon the ferroelectric capacitors within the cell.

The disadvantage of the above approach is that Eaton Jr. requires thateach cell contain at least two transistors and two ferroelectriccapacitors. This approach takes up a large area of the chip forimplementation, which limits the overall density of a memory array.

An improvement on the Eaton Jr. et al. approach is found in U.S. patentapplication Ser. No. 08/175,923 entitled "REFERENCE CIRCUIT FOR ANONVOLATILE FERROELECTRIC MEMORY" to Lowry et al. This patentapplication is assigned to the same assignee as the present patentapplication. It is not prior art. This improvement for ferroelectricmemory designs from Lowry et al. describes an array of memory cells inwhich each cell comprises a single ferroelectric capacitor and a singleaccess transistor. The cells are arranged in a regular array such thatcommon word lines and common plate lines are used to access rows offerroelectric capacitor cells. The Lowry et al. patent applicationdescribes a folded bit line architecture in which bit lines for adjacentcolumns have staggered cells, such that a word line (WL) from one rowdoes not activate memory cells 80 on adjacent bit lines, as seen in FIG.4. It will be appreciated that the plate lines PL0 and PL1 can becombined as one common plate. In the Lowry et al. patent application,the result is an active bit line (BL) for one column, and an inactivebit line for an adjacent column when the word line and plate lines areactivated for reading or writing a particular ferroelectric memory cell.With an adjacent inactive bit line, the unused bit line is available forattachment to a single-ended reference circuit 90 using a referencedecoder 92. The single-ended reference circuit of Lowry et al. allowsfor placing a highly accurate voltage on the adjacent unused bit line,which can be used by the sense amplifier 94 to compare to any active bitline to determine the state of the ferroelectric capacitor, when theplate line is pulsed. U.S. patent application Ser. No. 08/175,923 toLowry et al. is hereby incorporated by reference.

Still lacking in the industry is a ferroelectric capacitor cell memoryarray using a RAM architecture which can be read using a sense amplifiercircuit and a reference voltage, where the reference voltage can beadjusted during fabrication and is substantially unaffected byfluctuations in external supply voltage.

SUMMARY OF THE INVENTION

The present invention solves the aforementioned problems in theindustry, and other problems, which will be appreciated by those skilledin the art upon reading and understanding the following specification.The present invention describes a circuit and method of implementing aferroelectric memory cell in a regular array and use of reference cellsto generate a reference voltage which is compared to the memory cellusing a multiplexed sense amplifier. Using a non-remnant capacitor, asingle-ended reference voltage can be generated on a reference bit line.A cell can then be read by comparing the voltage on its correspondingbit line to the reference bit line using the sense amplifier.

In particular, the present invention describes an integrated circuitmemory comprising an array of ferroelectric memory cells, a senseamplifier to sense and amplify data stored on the ferroelectric memorycells, a bit line for coupling the ferroelectric memory cells to thesense amplifier, and reference circuitry coupled to the sense amplifierhaving a non-remnant capacitor circuit for providing a referencevoltage. The reference circuitry can comprise a voltage regulatorconnected to a supply voltage and selectively coupled to the non-remnantcapacitor circuit, and isolation circuitry located electrically betweenthe non-remnant capacitor circuit and the sense amplifier.

In another embodiment, the integrated circuit memory can further includea coupling transistor connected to the non-remnant capacitor and thereference bit line for selectively coupling the non-remnant capacitor tothe reference bit line. Additionally, integrated circuit memory caninclude a coupling transistor connected to the voltage regulator and thenon-remnant capacitor circuit for selectively coupling the voltageregulator the capacitor circuit. Finally, another embodiment allows thenon-remnant capacitor to be adjusted.

In another embodiment, a method of generating a reference voltage in aferroelectric memory circuit is described. The method comprises thesteps of charging a non-remnant capacitor circuit to a pre-determinedvoltage, and sharing the charge stored on the non-remnant capacitorcircuit with a bit line, where a capacitance of the bit line is greaterthan the capacitance of the non-remnant capacitor. In one embodiment,the capacitance of the bit line is a magnitude greater than thecapacitance of the non-remnant capacitor.

In yet another embodiment, a method of reading a ferroelectric memorycomprises a plurality of ferroelectric memory cells, and a senseamplifier is described. The method comprises the steps of selectivelycoupling one of the ferroelectric memory cells to the sense amplifier,raising a voltage of a plate line of the ferroelectric memory cells toextract data stored in the ferroelectric memory cells, producing areference voltage using a non-remnant capacitor, and electricallycoupling the reference voltage to the sense amplifier. The step ofproducing the reference voltage can comprise the steps of charging thenon-remnant capacitor to a pre-determined voltage, and sharing thecharge stored on the non-remnant capacitor with a reference bit line,where a capacitance of the reference bit line is greater than thecapacitance of the non-remnant capacitor.

Finally, in another embodiment, a method is described for testing a cellmargin in a ferroelectric memory comprising a plurality of ferroelectricmemory cells, and a sense amplifier. The method comprises the steps ofselectively coupling one of the ferroelectric memory cells to the senseamplifier, the one of the ferroelectric memory cells storing datarepresented by a high voltage. A reference voltage is produced using anon-remnant capacitor, and electrically coupled to the sense amplifier.The data stored in the one of the ferroelectric memory cells is readusing the sense amplifier, and the reference voltage is adjusted untilthe data stored in the one of the ferroelectric memory cells is readerroneous.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, where like numerals refer to like components throughoutthe several views,

FIG. 1 is a hysteresis curve of a ferroelectric capacitor;

FIG. 2 is a hysteresis curve of an aged ferroelectric capacitor;

FIG. 3 is a ferroelectric memory having two capacitors per memory cell;

FIG. 4 is a portion of a ferroelectric memory array having a folded bitline architecture and single-ended reference circuit;

FIG. 5 is a block diagram of a memory incorporating the presentinvention;

FIG. 6 is a portion of a ferroelectric memory array having a folded bitline architecture and single-ended reference circuit of FIG. 5;

FIG. 7 is a timing diagram of a clocked plate method of accessing aferroelectric memory cell;

FIG. 8 is a timing diagram of a pulsed plate method of accessing aferroelectric memory cell;

FIGS. 9 and 10 are schematic diagrams of a pair of bit lines andassociated circuitry of the memory of FIG. 5;

FIG. 11 is a timing diagram illustrating the operation of the referencecell circuit of the schematic of FIGS. 9 and 10; and

FIG. 12 is a timing diagram illustrating the operation of the referencecharge circuit of the schematic of FIGS. 9 and 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following detailed description of the preferred embodiments,reference is made to the accompanying drawings which form a part hereof,and in which is shown by way of illustration specific preferredembodiments in which the inventions may be practiced. These embodimentsare described in sufficient detail to enable those skilled in the art topractice the invention, and it is to be understood that otherembodiments may be utilized and that logical, mechanical and electricalchanges may bemade without departing from the spirit and scope of thepresent inventions.The following derailed description is, therefore, notto be taken in a limiting sense, and the scope of the present inventionsis defined only bythe appended claims.

This invention concerns electrical circuitry which uses voltages torepresent the two binary logic levels. The words "low" and "high" inthis specification refer generally to the false and true binary logiclevels, respectively. Signals are generally considered active when theyare high, however, an asterisk (*) following the signal name, or a barabove the signal name in this application indicates that the signal isnegative or inverse logic. Negative or inverse logic is consideredactive when the signal is low.

FIG. 5 shows a general block diagram of a memory circuit 100incorporating the present invention. The memory circuit is coupled to amicroprocessor 101 and includes a memory array 102, column 104 and row106 decoders, and a control circuit 108. The memory circuit alsoincludes input 110 and output 112 buffers connected to data input anddata output lines, respectively. The data input and output lines can bemultiplexed together,but have been illustrated separately forsimplicity. Address lines 113 are provided as input to the row andcolumn decoders to address a portion of the memory array.

In operation, the memory circuit control 108 responds to control inputs109from the microprocessor to perform different operations on the memoryarray. In particular, the control circuit is used to read data from andwrite data to the memory array 102. During one of these accessoperations,an address provided on the address lines 113 is decoded bythe row decoder 106 to access one row of the memory array. Likewise,input provided on theaddress lines is decoded by the column decoder 104to access at least one column of the memory array. During a readoperation, the data stored in the addressed memory cell(s) is thentransferred to the output buffer 112 and provided on the data outputlines. In a write operation, the addressedmemory cell is accessed anddata provided on the data input lines is storedin the cell.

FIG. 6 illustrates the basic concept of a portion of a ferroelectricmemoryarray shown in FIG. 4, a detailed description of array 102 isprovided below with reference to FIGS. 9 and 10. Two communicationlines, Bit line 1 and Bit line 2, couple ferroelectric memory cellcapacitors 116 to the data I/O lines which are connected to input andoutput buffers. The memorycells have one plate connected to a plate lineand the other plate connected to an n-channel MOS access transistor 118.The gate of the access transistor is connected to a word line. The wordlines and access transistors are arranged so that one word line couplesa memory cell 116(1) to Bit line 1, and the next word line couples amemory cell 116(2) to the Bit line 2.

To access a memory cell, the external address lines 113 are decoded toidentify which row of the memory array is to be accessed. Thecorresponding word line is then raised to a high voltage level so thatallof the access transistor having their gate connected to that wordline are activated. All of the memory cells associated with that wordline, therefore, are coupled to one of the bit lines. If the memory cellwere a standard DRAM capacitor, the charge stored on the memory cellcapacitors would be shared with the bit lines when the access transistor118 is activated. Likewise, a ferroelectric memory cell having somenon-remnant charge stored thereon will produce a small change to the bitline voltage.If the non-remnant component of the ferroelectric memorycell has been discharged, the bit line voltage will not change. Toaccess the data stored in the memory cell, the plate line associatedwith the addressed row is pulsed high. As known to one skilled in theart, if the polarization of the ferroelectric dielectric is in one stateand the plateline is pulsed high, the current through the memory cell116 will increase the voltage on the bit line which is coupled thereto.If the polarization of the memory cell is in the opposite state, currentthrough the memory cell is resisted and the voltage of the coupled bitline does not change.

The change in the voltage on the bit line can be sensed using a senseamplifier circuit and a reference circuit 120 which produces a referencevoltage. The sense amplifiers are typically comprised of both an n-sense122 and a p-sense 123 amplifier and detect the difference in the voltageon the bit line and the reference circuit voltage coupled to the otherbitline via coupling transistor 124, as known to one skilled in the art.The sense amplifiers drive the bit line pairs to the appropriate voltagerail.That is, if the voltage of the bit line is higher than thereference circuit voltage, the bit line is driven to the positivevoltage rail. Conversely, if the bit line is lower than the referencecircuit, the bit line is driven to ground. It will be recognized thatdifferent sense amplifier and reference circuits can be used withoutdeparting from the spirit of the present invention. A reference decodecircuit 125 is provided to selectively connect the reference voltage tothe unused bit line. By reading the memory cell, the state of thepolarization may be reversed and thus the data which was stored on thecell will be revered. That is, in the preferred embodiment thepolarization of the dielectric issuch that a logical "one" provides achange in voltage of the digit line. To provide the requisite voltagechange on the bit line, the polarization of the dielectric is changed.To read a logical one, therefore, the polarization of the memory cell ischanged by reading the cell and will represent a logical "zero" afterthe cell has been read. If, on the other hand, the memory cell wasstoring a zero, there will be no voltage change on the bit line and thememory cell will remain polarized in a zero state.It can be seen that bymerely reading a memory cell the data will be lost if the memory cellwas storing a one. A write-back operation must be performed on thememory cell so that the polarization of the memory cell is returned toits pre-read state.

To write a one to the memory cell 116, the bit line coupled to thememory cell must be raised to the positive voltage rail while the plateline is at a low potential. This reverses the polarization of theferroelectric dielectric which resulted from pulsing the plate linehigh. The sense amplifiers 122 and 123 are used to write-back a logicalone to a memory cell from which a one was read. All of the senseamplifiers associated with a row of memory cells in the memory array arestrobed at once. If thecolumn address, therefore, is changed during theread operation, the data stored in any one of the memory cells of a rowwill not be lost. If, however, the row address is changed during theread operation prior to thesense amplifiers driving the active bit lineto a full positive voltage rail, the access transistor 118 connected tothe addressed memory cell will be turned off and the data cannot bere-written to the memory cell.

It will be helpful to fully understand the effect of the voltage placedon the memory cell capacitor plates. FIGS. 7 and 8 illustrate twoalternate ways of reading a ferroelectric memory and restoring the dataread therefrom. The fast method is referred to herein as a clocked plateline, as illustrated in FIG. 7. To access a memory cell, a word line isselectively raised. The plate line of the memory cell is then raised andthe data stored on the cell is coupled to the bit line as describedabove.If the memory cell was storing a 1, the memory cell was rewrittento a 0 merely by reading the cell. The sense amplifiers are then fired,at reference number 155, so that the bit lines are driven to fullvoltage rails. Because the plate line is high when the bit lines aredriven to rail, a 1 is not restored on the memory cell. A 1 will berestored when the plate line goes low while the bit line is high. Itwill be understood that the bit line potentials can be reversed tochange the data on the memory cell and that a 0 will be written to thecell when the bit line is low while the plate line is high.

The second method of reading and restoring a memory cell is referred toas a pulsed plate line and illustrated in FIG. 8. To read the cell, theplateline is pulsed high twice while the cell is coupled to a bit line.The datastored on the memory cell will be reflected on the bit lineafter the firstpulse and the bit line pair is then driven to voltagerails, at reference number 155. If the bit line coupled to the memorycell goes high while theplate line is low, the memory cell will bewritten to a 1. If the potentialon the bit lines are reversed to changethe data on the memory cell, a 0 cannot be written to the cell until theplate line is pulsed high again. The plate line is therefore pulsed higha second time to insure that the memory cell stores the proper data.

Because the polarization of the ferroelectric memory cell is determinedby the potential of both of the cell's plates at any given time. Thatis, a 1is stored anytime the bit line is high while the plate line islow, and a 0is written anytime the plate line is high while the bit lineis low. If both plates are at the same potential, the data stored on thecell is not changed. It can be appreciated that the docked plate linemethod is an efficient way of reading from and writing new data to amemory cell, and that the pulsed plate line method is an efficient wayof reading and restoring data from a memory cell. Both methods areinterchangeable, but the pulsed method is used herein to detail thepresent invention.

Reference Voltage Circuit

Referring to FIGS. 9 and 10 a portion of the memory array 102 of thememorycircuit of the present invention shown in FIG. 5 is described indetail. Asstated above, the memory array is arranged in addressable rowsand columns,and a memory cell is located at the intersection of each rowand column. One memory cell can therefore be accessed by selecting botha row and a column. The memory array is, conceptually, arranged as aplurality of memory cell planes having common rows. This arrangementallows one memory cell in each plane to be accessed by selecting one rowand addressing a column in each plane. For example, by using eightplanes of memory cells, one byte of data can be stored to, or retrievedfrom, the array by addressing one row and one column of the array. Eachmemory array plane, therefore, has a separate input/output connection.

In the present invention, each column of a plane of memory is arrangedas apair of folded bit lines 150 and 152. The bit lines can beelectrically separated into sections using isolation transistors, andare represented in FIGS. 9 and 10 by three sections (a), (b), and (c)Each column contains1024 addressable memory cells with each one capableof storing one bit of data. These memory cells are arranged in two512-bit sub-arrays 154 and 156. Each sub-array therefore has 512 memorycells which can be coupled tothe bit lines. The memory cells are eachcomprised of a single ferroelectric capacitor 158 which has one plateconnected to a plate line 162 and a second plate connected to an-channel access transistor 160. Thegates 161 of the access transistorsare connected to word lines. Each word line is associated with a row ofthe memory array such that by addressing one row of the array, one wordline is raised to activate one access transistor in each column. Bitlines 150 and 152 each have an I/O transistor 164 which are used toselect the column of the memory array to access via the datainput/output lines.

The bit lines are connected to a p-sense amplifier 168, as known to oneskilled in the art, located between the sub-arrays 154 and 156. Thep-sense amplifier is used to sense a differential voltage between thebit lines and drive the bit line with the higher voltage to a full powersupply positive rail. An n-sense amplifier 170 located between sub-array156 and the I/O transistors 164 is also connected to both bit lines. Then-channel sense amplifier, as known to one skilled in the art, is usedto sense a differential voltage between the bit lines and drive the bitline with the lower voltage to the full negative voltage rail, typicallyground.

In general, to read data from a memory cell, the bit lines areequilibratedto a predetermined level. The memory cell is then coupled toone of the bitlines by activating the gate of its access transistor.Once the memory cellis coupled to a bit line, the plate line is pulsedhigh such that a voltageis imparted on the bit line if the memory cellwas storing one data logic state, for example a 1. If the memory cellwas storing a 0, the voltage onthe digit line would remain at theequilibrated level. As explained below, the voltage generated by readinga logic 1 is small and must be amplified before coupling the bit linesto the I/O lines. To amplify the bit line coupled to the selected memorycell, a reference voltage is coupled to theother bit line prior toactivating the p-sense amplifier and the n-sense amplifier. Thereference voltage is a voltage between the equilibrate level and thevoltage created by reading a logic 1. Two different reference circuitsand methods are described herein for producing the reference voltage.

The first reference circuit is comprised of two pairs of ferroelectricmemory cells 172(a) and (b). Each pair is located beside one of thesub-arrays and on opposite sides of the p-sense amplifier 168. Each oftheferroelectric capacitors 173 of a reference cell pair stores adifferent logic state. That is, one of the capacitors stores a logic 1,while the other capacitor stores a logic 0. By coupling the bit linestogether afterthey have been coupled to the capacitors, a referencevoltage can be generated which has a voltage level that is half thelevel of a logic 1 voltage.

Some of the other circuitry illustrated includes equilibrate transistors186, 188, 190, and 192. Isolation transistors 176 and 194 are used tocouple the n-sense amplifier to either the bit lines 150 and 152, ortransistors 164. Isolation transistors 178, 180, 182, and 184 areprovidedto selectively couple two of the bit lines 150(a), 150(b),152(a), or 152(b) to the p-sense amplifier. Transistors 174 are used tocouple the n-sense amplifier to ground and transistor 192 is used toequilibrate the n-sense amplifier.

To fully understand how the reference circuit is used to read a memorycell, reference is made to the timing diagram of FIG. 11. To read datastored in a memory cell which is to be coupled to bit line 150(a), thebitlines must be equilibrated to a low voltage level. N-channeltransistors 174 are, therefore, activated so that bit lines 150 and 152are pulled low. By activating isolation transistors 176, 178, 180, 182and 184 while the transistors 174 are activated, all three sections ofthe bit lines arepulled low. Equilibrate transistors 186 and 190 areactivated to insure that both bit lines are at equal voltage levels.Transistors 174 and 176 are then deactivated to decouple the bit linesfrom ground, and isolate section (c) of the bit lines, respectively.Transistor 178 is mined off toisolate bit line 150(b) from senseamplifier 168. Similarly, transistor 184is turned off to isolate bitline 152(a) from sense amplifier 168. Isolation transistors 180 and 182remain activated.

Once the appropriate sections are isolated from the p-sense amplifier168 aword line can be selectively raised to couple a memory capacitor158 to bitline 150(a). Although the memory cell described herein iscoupled to bit line 150(a), it will be recognized that any of the memorycells can be read by selectively controlling isolation transistors 178,180, 182 and 184. Equilibrate transistors 186 and 190 are then turnedoff. As stated above, the voltage on the bit line will not change untilthe plate line ispulsed. While the memory cell plate line is pulsed, thevoltage on the bit line will increase. The bit line will remain high ifthe capacitor is storing a logic 1. If the capacitor is storing a logic0, the bit line will return low when the plate line returns low. On theopposite side of the p-sense amplifier, the two reference cellcapacitors 173 of reference circuit 172(b) are coupled to bit lines150(b) and 152(b) via transistors 163. The reference cell plate linesare pulsed at the same time the memorycell plate lines are pulsed. Forclarification, the bit lines which are coupled to the reference cellcapacitors are referred to herein as reference bit lines. Because thereference cell capacitors are storing opposite logic levels, only onebit line will increase in voltage level.

Activating equilibration transistor 190 while the plate lines are beingpulsed couples bit lines 150(b) and 152(b) together and shares theindividual line voltages such that a voltage level is created that isbetween the equilibrate voltage level (ground) and the logic 1 level. Itwill be appreciated that by using ferroelectric capacitors, thereference voltage accurately tracks the voltage created by theferroelectric memory cells.

To sense the data stored in the accessed memory cell, the p-senseamplifieris fired so that the bit line, 150(a) or 152(b), with thehighest voltage level is pulled to a high voltage level. Isolationtransistors 176, 178 and 184 are then activated to couple the bit linesto the n-sense amplifier 170. The n-sense amplifier is then fired todrive the bit line, 150(c) or 152(c), with the lowest voltage level toground.

As explained above, a memory cell which had a logical 1 stored thereonwas re-written to a logical 0 merely by reading the memory cell. Thememory cell will be returned to a logical 1, however, when the bit lineis drivenhigh while the memory cell plate line is low. The pair ofreference circuitcapacitors will also be restored to opposite states onthe same cycle as the accessed memory cell. It is preferred to pulse theplate lines high a second time after both sense amplifiers have beenfired so that the bit lines are at full voltage rails. It will berecognized that the reference cell plate line and the memory cell plateline can be pulsed together. Thereference circuit capacitor 173 coupledto the low bit line will be writtento a 0 and the capacitor 173 coupledto the high bit line will remain written to a 1. It is also preferred todischarge the memory capacitors 158 and the reference capacitors 173prior to turning access transistors 160 and 163 off to reduce fatiguecaused by non-remnant charges. Because the capacitors can be written toa logical 1 by raising the voltage on thebit line, traditionalequilibration techniques cannot be used. That is, coupling the bit linestogether will lower one bit line while raising the other. Transistors174 are therefore used to pull both bit lines to groundto discharge thecells without effecting the data stored thereon.

The above described reference circuit which uses a pair of ferroelectricmemory cells to generate the reference voltage can become fatigued withtime. The advantage of having a reference voltage generated byferroelectric capacitors which perform in the same manner as the memorycells is tainted by the possibility that the reference voltage maychange over time. That is, every time a ferroelectric cell is fired, theperformance of the material is slightly degraded. Because the referencecells 173 are fired substantially more often than the memory cells 158,there is a possibility that the reference cells will degrade faster andthe resultant reference voltage will change over time. To insure thatthe reference voltage is stable and located between the voltagesresulting from the two logic states, a reference circuit is providedwhich stores a non-remnant charge on a reference capacitor.

The reference charge circuit 196 includes at least one ferroelectriccapacitor 198. Although the capacitor is ferroelectric, it is operatedas a standard non-remnant capacitor. The capacitor can be selectivelycoupledto either of the bit lines 150 or 152 by activating eitherisolation transistor 204 or 206. Transistors 202 and 200 can beactivated to couple the capacitor 198 to a reference voltage, Vref,provided by a voltage regulator 208. In theory, a voltage regulatorcircuit could be used to provide the reference voltage directly,however, changes in the external power supply voltage would effect theregulated voltage level. Changes in the reference voltage could resultin the inability to read data stored ina memory cell. This is becausethe voltage produced by the ferroelectric memory cell is not dependentupon the supply voltage and is therefore not effected by changes in thesupply voltage.

The ferroelectric capacitor 198 and the capacitance of line 203 have asubstantially lower combined capacitance than the line capacitance ofeither bit line 150 or 152. It is preferred that the referencecapacitancebe about 1/10 the size of the bit lines. This capacitanceratio allows the reference capacitor to be charged to Vref and thenshared with the bit line capacitance. Thus, charge conservation definedby C1*V1+C2*V2=C_(FINAL) V_(FINAL) where C1 is the bit line capacitanceand C2 is the reference capacitance, can be used to determine the finalvoltage on the bit line after the reference capacitor has been coupledthereto. For example, if the reference capacitor has been charged to 4volts and the bit line has been equilibrated to 0 volts, the final bitline voltage after capacitor 198 is coupled to the bit line is definedas:

    V.sub.FINAL =C2*4/C.sub.FINAL,

or

    V.sub.FINAL =4/10=0.4 volts

It can be seen that if the reference voltage fluctuates with changes inthesupply voltage, the final bit line voltage will only change by 1/10of the fluctuation. The reference voltage, therefore, is substantiallysupply voltage independent.

Capacitor 198 can be a plurality of parallel capacitors such that alaser or electronic key, as known to one skilled in the art, can then beused to "trim" the capacitance so that the final reference voltage levelon the bit line is between ground and the voltage generated by a memorycell storing a logic 1. Further, the reference capacitor is not limitedto ferroelectric capacitors but can be any non-remnant capacitor.

A course adjustment of the reference voltage can be made by adjustingthe regulated voltage, while a fine adjustment can be made by changingthe number of capacitors 198 coupled to line 203. By changing theregulated voltage cell margin can be measured for the memory circuit.This is accomplished by systematically raising the reference voltageuntil a memory cell storing a logical 1 is read as a logical 0. That is,when a memory cell is read incorrectly the reference voltage is abovethe memory cell voltage. The difference between the beginning referencevoltage and the voltage at which the memory cell is erroneously read isthe cell margin.

In operation, a memory cell is read in a similar manner to thatdescribed above using the reference cell circuit. Referring to FIG. 12,to read datastored in a memory cell which is to be coupled to bit line150(a), the bit lines must be equilibrated to a low voltage level.N-channel transistors 174 are, therefore, activated so that bit lines150 and 152 are pulled low. By activating isolation transistors 176,178, 180, 182 and 184 while the transistors 174 are activated, all threesections of the bit lines arepulled low. Equilibrate transistors 186 areactivated to insure that both bit lines are at equal voltage levels.Transistors 174 and 176 are then deactivated to decouple the bit linesfrom ground, and isolate section (c)of the bit lines, respectively.Transistor 178 is turned off to isolate bitline 150(b) from senseamplifier 168. Similarly, transistor 180 is turned off to isolate bitline 152(b) from sense amplifier 168. Isolation transistors 182 and 184remain activated so that section (a) of the bit lines are coupled to thep-sense amplifier.

Equilibrate transistors 204 and 206 are turned off so that node 203 isisolated from section (a) of the bit lines. Transistor 202 has beenturnedon so that node 203 and any activated capacitors 198 are chargedto the voltage level of the regulator circuit 208. Transistor 202 isthen turned off and a word line, or gate of transistor 161, can beselectively raised to couple a memory capacitor 158 to bit line 150(a).Although the memory cell described herein is coupled to bit line 150(a),it will be recognizedthat any of the memory cells can be read byselectively controlling isolation transistors 178, 180, 182 and 184.Equilibrate transistors 186 are then turned off. As stated above, thevoltage on the bit line will notchange until the plate line is pulsed.The plate line is pulsed high to read a memory cell. The memory cellillustrated in FIG. 12 is storing a logical 1, such that bit line 150(a)maintains a high voltage after the plate line returns low. The gate oftransistor 206 is also pulsed high when the plate line is pulsed so thatthe charge stored on line 203 is shared with bit line 152(a). By chargesharing line 203 with 152(a), the voltage of line 152(a) is raised to areference level which is 1/10 the level of line 203.

The p-sense amplifier is fired so that the bit line, 150(a), which hasthe highest level is driven to the full positive voltage rail.Transistors 178and 180 activated to couple the p-sense amplifier to then-sense amplifier,and then the n-sense amplifier is then fired so thatbit line 152 is drivento ground. After line 203 is charge shared with abit line, transistor 202 is re-activated to couple the line to thevoltage regulator and re-charge both line 203 and capacitors 198.

It will be appreciated that both the reference cell circuit 172 and thereference charge circuit can be included in one memory circuit toprovide flexibility and redundancy.

CONCLUSION

A ferroelectric memory circuit has been described which has an array offerroelectric memory cells fabricated as capacitors. The memory cellscan be selectively coupled to a bit line for writing to or readingtherefrom. Sense amplifiers have been described for reading the memorycells by sensing a voltage on the bit line resulting from thepolarization of the memory cell coupled thereto. A reference voltage isused by the sense amplifiers to determine the state of the data storedon a memory cell. Thereference voltage is produced using a non-remnantcapacitor circuit. A circuit and method have been described for sharinga reference charge stored on the non-remnant capacitor circuit with areference bit line. Thecapacitance of the bit line is about ten timesthat of the non-remnant capacitor. The resultant voltage imparted on thereference bit line, therefore, is 1/10 the voltage used to charge thenon-remnant capacitor circuit and is substantially unaffected byfluctuations in the supply voltage. The capacitance of the non-remnantcapacitor circuit can be adjusted during fabrication to "fine tone" thereference voltage. A circuit and method have been described forselectively coupling the reference voltage to the sense amplifiers basedon the location of the memory cell being read. Further, a test method isdescribed for determining the cell margin of the memory circuit byadjusting the reference voltage.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. Therefore, it is manifestly intended that this invention belimited only by the claims and the equivalents thereof.

What is claimed is:
 1. An integrated circuit memory comprising:an arrayof ferroelectric memory cells; a sense amplifier to sense and amplifydata stored on the ferroelectric memory cells; first and second bitlines for coupling the ferroelectric memory cells to the senseamplifier; and reference circuitry selectively coupled to either thefirst or second bit line, the reference circuitry having a non-remnantcapacitor circuit for providing a reference voltage, the non-remnantcapacitor circuit is comprised of a conductor line having a linecapacitance, and a plurality of capacitors selectively coupled to theconductor line via access devices.
 2. A method of generating a referencevoltage in a ferroelectric memory circuit, the method comprising thesteps of:charging a non-remnant capacitor circuit to a pre-determinedvoltage; and sharing the charge stored on the non-remnant capacitorcircuit with a bit line,where a capacitance of the bit line is an orderof magnitude greater than the capacitance of the non-remnant capacitor.3. A method of testing a cell margin in a ferroelectric memorycomprising a plurality of ferroelectric memory cells, and a senseamplifier, the method comprising the steps of:(a) selectively couplingone of the ferroelectric memory cells to the sense amplifier, the one ofthe ferroelectric memory cells storing data represented by a highvoltage; (b) producing a reference voltage using a non-remnantcapacitor; (c) electrically coupling the reference voltage to the senseamplifier; (d) reading the data stored in the one of the ferroelectricmemory cells using the sense amplifier; and (e) adjusting the referencevoltage and repeating steps (a)-(d) until the data stored in the one ofthe ferroelectric memory cells is read erroneous.
 4. An integratedcircuit memory comprising:a sense amplifier having first and secondsensing nodes; a first pair of bit lines connected to the first andsecond sensing nodes of the sense amplifier; a second pair of bit linesconnected to the first and second sensing nodes of the sense amplifier;ferroelectric memory cells coupled to the first and second pair of bitlines; a reference line having a capacitance; a plurality of capacitorscoupled to the reference line through access transistors; and isolationdevices connected to the reference line and the second pair of bit linesfor coupling the reference line to either the first or second referencenode of the sense amplifier via the second pair of bit lines.
 5. Theintegrated circuit memory of claim 4 wherein a capacitance of a bit lineof the second bit line pair is an order of magnitude greater than a sumcapacitance of the reference bit line and the plurality of capacitors.6. The integrated circuit memory of claim 4 further comprising anelectronic key for activating the access transistors to selectivelycouple the plurality of capacitors to the reference line.
 7. A method ofreading a ferroelectric memory comprising a plurality of ferroelectricmemory cells, first and second bit lines, and a sense amplifier, themethod comprising the steps of:equilibrating the first and second bitlines to a ground potential; charging a non-remnant capacitor to apre-determined positive voltage simultaneously with the equilibratingstep; selectively coupling one of the ferroelectric memory cells to thesense amplifier via the first bit line; raising a voltage of a plateline of the one of the ferroelectric memory cells to extract data storedin the ferroelectric memory cells; sharing the charge stored on thenon-remnant capacitor with the second bit line, where a capacitance ofthe second bit line is greater than the capacitance of the non-remnantcapacitor; and electrically coupling the reference voltage to the senseamplifier via the second bit line.